This invention relates to a gas detecting element, and more particular, is directed to a gas detecting element with a catalyst layer.
Various researches have developed a gas detecting element using a gas sensitive body, such as SnO.sub.2 type oxide. The resistance value of the body changes when it comes into contact with gases. Such a gas detecting element employs a catalyst in order to enhance its detecting sensitivity. In one example of a gas detecting element, the catalyst layer is provided on the gas sensitive body. The gas detecting element, the material of the gas sensitive body and the catalyst layer are generally selected in order that the gas detecting element has a specific sensitivity for one particular gas, such as CO gas. The catalyst layer is a thick film formed by a mixture of a carrier material with a noble metal such as platinum (Pt), Palladium (Pd). In a gas detecting element employing the catalyst layer described above, the sensitivity of the element is enhanced, but the sensitivity is adversely affected by humidity. The sensitivity deteriorates after the element is kept under high humidity for a long time. Where the gas detecting element is used for detecting a dangerous gas, such as CO gas, the defects of the element described above becomes a very serious problem. Thus, a gas detecting element unaffected by humidity is greatly desired.
With the present trend toward the use of more and more electronics in residential homes, many homes now use a ventilation system that automatically ventilates a room by a control signal from a gas detecting element. The gas detecting element which is used for the ventilation system should have the following characteristics.
(1) The gas detecting element should be highly sensitive to all gases which are dangerous or iritating to humans. PA1 (2) The gas detecting element should have a quick response characteristic and a quick release characteristic. PA1 (3) The gas detecting element should have an excellent resistance to humidity.
If a gas detecting element has high sensitivity to only one kind of gas, it is difficult to establish a threshold value for generating switching signals. Thus, a gas detecting element should have uniform sensitivity to all types of gases to be detected. When the concentration of the gas to be detected exceeds a predetermined concentration, the gas detecting element should quickly change its resistance value so as to generate a control signal. Likewise, when the concentration of the gas to be detected decreases below a predetermined concentration, the gas detecting element should quickly return to its original resistance value.
It is also important that the gas detecting element have excellent resistance to humidity so that the element can be used under various environmental conditions. Furthermore, if the gas detecting element is not operated continuously, it is also desirable that the element quickly return to a stable state when it is again used.
The gases to be detected by the gas detecting element for ventilation purposes include CO (carbon monoxide) gas, tobacco smoke, alcohol, etc. The conventional gas detecting element employing a noble metal, such a plantinum (Pt), palladium (Pd), etc. as a catalyst material, does not have sufficient desirable detecting characteristics for a ventilation system because the sensitivity of the gas detecting element to tobacco smoke is very low. Moreover, the element does not exhibit good resistance to humidity.
Japanese patent disclosure No. 58-118953, which was published on July 15, 1983, discloses a gas detecting element having a gas sensitive body mixed with a catalyst material consisting of a carrier material of TiO.sub.2 and a catalyst material of W (tungsten) in order to enhance its sensitivity for detecting propane gas, gas consisting of methane gas as a main component and CO (carbon monoxide) gas. The element is deficient, however, with respect to uniformity of sensitivity, humidity resistance, and stability.